Method for high aspect ratio HDP CVD gapfill

ABSTRACT

A method of depositing a high density plasma silicon oxide layer having improved gapfill capabilities. In one embodiment the method includes flowing a process gas consisting of a silicon-containing source, an oxygen-containing source and helium into a substrate processing chamber and forming a plasma from the process gas. The ratio of the flow rate of the helium with respect to the combined flow rate of the silicon source and oxygen source is between 0.5:1 and 3.0:1 inclusive. In one particular embodiment, the process gas consists of monosilane (SiH 4 ), molecular oxygen (O 2 ) and helium.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/137,132, filed Apr. 30, 2002, entitled “METHOD FOR HIGH ASPECT RATIOHDP CVD GAPFILL,” having Zhong Qiang Hua et al. listed as inventors. The10/137,132 application is assigned to Applied Materials, Inc., theassignee of the present invention and is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

One of the primary steps in the fabrication of modern semiconductordevices is the formation of a film, such as a silicon oxide, on asemiconductor substrate. Silicon oxide is widely used as an insulatinglayer in the manufacture of semiconductor devices. As is well known, asilicon oxide film can be deposited by thermal chemical vapor deposition(CVD) or a plasma chemical vapor deposition processes among othertechniques. In a conventional thermal CVD process, reactive gases aresupplied to the substrate surface where heat-induced chemical reactions(homogeneous or heterogeneous) take place to produce a desired film. Ina conventional plasma process, a controlled plasma is formed todecompose and/or energize reactive species to produce the desired film.

Semiconductor device geometries have dramatically decreased in sizesince such devices were first introduced several decades ago. Smallerfeature sizes have resulted in the presence of increased aspect ratiogaps for some applications, for example, between adjacent conductivelines or in etched trenches. The aspect ratio of a gap is defined by theratio of the gap's height or depth to its width. These spaces aredifficult to fill using conventional CVD methods. A film's ability tocompletely fill such gaps is referred to as the film's “gapfilling”ability. Silicon oxide is one type of insulation film that is commonlyused to fill the gaps in intermetal dielectric (IMD) applications,premetal dielectric (PMD) applications and shallow trench isolation(STI) applications among others. Such a silicon oxide film is oftenreferred to as a gapfill film or a gapfill layer.

Some integrated circuit manufacturers have turned to the use of highdensity plasma CVD (HDP-CVD) systems to deposit silicon oxide gapfilllayers. HDP-CVD systems form a plasma that is approximately two ordersof magnitude or greater than the density of a standard,capacitively-coupled plasma CVD system. Examples of HDP-CVD systemsinclude inductively-coupled plasma systems and electron cyclotronresonance (ECR) plasma systems among others. HDP-CVD systems generallyoperate at lower pressure ranges than low density plasma systems. Thelow chamber pressure employed in HDP-CVD systems provides active specieshaving a long mean-free-path and reduced angular distribution. Thesefactors, in combination with the plasma's density, contribute to asignificant number of constituents from the plasma reaching even thedeepest portions of closely spaced gaps, providing a film with improvedgapfill capabilities as compared to films deposited in a low densityplasma CVD system.

Another factor that allows films deposited by HDP-CVD techniques to haveimproved gapfill characteristics as compared to films deposited by otherCVD techniques is the occurrence of sputtering, promoted by the plasma'shigh density, simultaneous with film deposition. The sputtering elementof HDP deposition slows deposition on certain features, such as thecorners of raised surfaces, thereby contributing to the increasedgapfill ability of HDP deposited films. Some HDP-CVD systems introduceargon or a similar heavy inert gas to further promote the sputteringeffect. These HDP-CVD systems typically employ an electrode within thesubstrate support pedestal that enables the creation of an electricfield to bias the plasma toward the substrate. The electric field can beapplied throughout the HDP deposition process to generate sputtering andprovide better gapfill characteristics for a given film.

One HDP-CVD process commonly used to deposit a silicon oxide film formsa plasma from a process gas that includes silane (SiH₄), molecularoxygen (O₂) and argon (Ar). The industry has found that silicon oxidefilm deposited according to such an HDP-CVD process are useful for avariety of applications and exhibit improved gapfill characteristics ascompared to many other silicon oxide film deposition techniques that donot rely on HDP-CVD technology. Recently, however, engineers havediscovered that for some high aspect ratio applications where the widthof a gap to be filled is in the range of 0.13 microns or less, theaddition of argon to the process gas actually hinders gapfillcapabilities. FIGS. 1A-1C, which are simplified cross-sectional views ofan HDP-CVD silicon oxide film at different stages of deposition, helpillustrate this problem. The HDP-CVD film formed in FIGS. 1A-1C wasdeposited in a Ultima™ HDP-CVD chamber manufactured by AppliedMaterials, the assignee of the present application, using the processset forth below in Table 1 which was optimized for gapfill properties.TABLE 1 PREVIOUSLY KNOWN HDP-CVD SiO₂ DEPOSITION PROCESS Parameter ValueSiH₄ flow 60 + 11 sccm O₂ flow 140 sccm Ar flow 80 + 12 sccm Pressure2-4 mTorr (TVO) Temperature 550° C. Top RF Power 4900 Watts Side RFPower 3000 Watts Bias RF Power 2000 Watts

For the gas flow entries within table 1 that include two numbers, thefirst number indicates the flow rate of the particular gas through sidenozzles of the HDP-CVD apparatus while the second number indicates theflow rate of the gas through a top, centered nozzle. Also, TVO means“throttle valve fully open” which results in chamber pressure beingcontrolled by the quantity of gas flowed into the chamber.

FIGS. 1A-1C, which are simplified cross-sectional views of a siliconoxide film at different stages of deposition, illustrate the potentialgapfill limitation that is associated with the process recipe of Table 1for certain small width gaps having relatively high aspect ratios. It isimportant to understand that while HDP-CVD silicon oxide depositiontechniques generally provide for improved gapfill as compared to otherplasma silicon oxide deposition techniques including low density,capacitively coupled plasma CVD techniques, the gapfill issuesassociated with those techniques become an issue for HDP-CVD techniquesin certain aggressive gapfill applications, for example, gaps having awidth of 0.10 μm and a 5:1 aspect ratio. The gapfill problem illustratedin FIGS. 1A-1C is not drawn to scale in order to more easily illustratethe problem.

FIG. 1A shows the initial stages of film deposition over a substrate(not shown) having a gap 120 defined by two adjacent features 122, 124formed over the substrate. As shown in FIG. 1A, the conventional HDP-CVDsilicon oxide deposition process results in direct silicon oxidedeposition on horizontal surface 126 within gap 120 and horizontalsurfaces 128 above features 122, 124. The process also results inindirect deposition (referred to as re-deposition) of silicon oxide onsidewalls 130 due to the recombination of material sputtered from thesilicon oxide film as it grows. In certain small-width,high-aspect-ratio applications, the continued growth of the siliconoxide film results in formations 132 on the upper section gap sidewallthat grow toward each other at a rate of growth exceeding the rate atwhich the film grows laterally on lower portions 134 of the sidewall(see FIG. 1B also). The final result of this process is that a void 136forms as shown in FIG. 1C.

One method that semiconductor manufacturers have developed in order toaddress this issue is to remove the argon from the process gasaltogether. Engineers at Applied Materials were able to develop anoptimized SiH₄ and O₂ HDP-CVD process without argon that was able toadequately fill gaps having an aspect ratio of 5:1 and a width of only0.15 microns. This SiH₄ and O₂ process, however, has so far proven to beinadequate at completely filling some even more aggressive gapfillapplications.

Accordingly, despite the improvement in gapfill capabilities provided byHDP-CVD systems and the relatively good gapfill characteristics ofHDP-CVD silicon oxide films in particular, the development of filmdeposition techniques that enable the deposition of silicon oxide layershaving even further improved gapfill characteristics are desirable. Suchimproved silicon oxide film deposition are particularly desirable inlight of the aggressive gapfill challenges presented by integratedcircuit designs employing minimum feature sizes of 0.10 microns andless.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention pertain to a method and apparatusfor depositing silicon oxide films having improved gapfill capabilities.Embodiments of the method of the invention deposit such films usingHDP-CVD deposition techniques and are particularly useful for premetaldielectric and shallow trench isolation applications in the manufactureof integrated circuits having minimum feature sizes of 0.10 microns orless. As used herein, a high density plasma is a plasma having an iondensity of at least 1×10¹¹ ions/cm³.

In one embodiment, the method includes flowing a process gas consistingof a silicon-containing source, an oxygen-containing source and heliuminto a substrate processing chamber and forming a plasma from theprocess gas. The inventors have found that adding helium to the processgas instead of argon actually improves gapfill capabilities of theprocess at such small minimum features sizes. In order to achieve theimproved gapfill capabilities, however, the inventors have found that isimportant to flow the helium source at a rate such that the ratio of theflow rate of helium with respect to the combined flow rate of thesilicon source and oxygen source is at least 0.5:1. According to someembodiments of the invention, the ratio of the flow rate of helium withrespect to the combined flow rate of the silicon source and oxygensource is between 0.5:1 to 3.0:1 inclusive. In one particularembodiment, the process gas consists of monosilane (SiH₄), molecularoxygen (O₂) and helium. Also, in some embodiments, the pressure levelwithin the chamber is maintained at or below about 7 mTorr.

According to another embodiment, a method of forming a shallow trenchisolation structure in a semiconductor substrate is disclosed. Themethod includes forming a silicon nitride layer over the substrate andetching a plurality of trenches in the substrate through the siliconnitride layer. Next, a silicon oxide layer is deposited over thesubstrate to fill the trenches by flowing a process gas consisting of asilicon source, an oxygen-containing source and helium into thesubstrate processing chamber and forming a high density plasma from saidprocess gas. The ratio of a flow rate of the helium source to a combinedflow rate of the silicon-containing source and the oxygen-containingsource in the process gas is between 0.5:1 and 3.0:1 inclusive.

These and other embodiments of the present invention, as well as itsadvantages and features are described in more detail in conjunction withthe text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are simplified cross-sectional views of an HDP-CVD siliconoxide film at different stages of deposition in a high aspect ratiogapfill application;

FIG. 2A is a simplified, cross-sectional view of an exemplary substrateprocessing system with which embodiments of the present invention may beused;

FIG. 2B is a simplified cross-sectional view of a gas ring that may beused in conjunction with the exemplary CVD processing chamber of FIG.2A;

FIG. 3 is a flowchart illustrating the steps according to one embodimentof the present invention; and

FIG. 4 is a simplified cross-sectional view of a shallow trenchisolation (STI) structure formed in a semiconductor substrate 200according to the process of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION I. INTRODUCTION

Embodiments of the present invention deposit a silicon oxide layer usinghigh density plasma CVD techniques. The deposited layer has improvedgapfill capabilities as compared to some prior art silicon oxidedeposition techniques. Films deposited by the method of the presentinvention are particularly suitable for use in the fabrication ofintegrated circuits having feature sizes of 0.10 microns or less.

Embodiments of the present invention can be implemented using a varietyof high density plasma CVD substrate processing chambers includingchambers in which a plasma is formed by the application of RF energy toa coil that at least partially surrounds a portion of the chamber andchambers that use ECR plasma formation techniques. An example of aninductively-coupled HDP-CVD chamber in which embodiments of the methodof the present invention can be practiced is set forth below.

II. EXEMPLARY SUBSTRATE PROCESSING SYSTEM

FIG. 2A illustrates one embodiment of a high density plasma chemicalvapor deposition (HDP-CVD) system 10 in which a dielectric layeraccording to the present invention can be deposited. System 10 includesa chamber 13, a substrate support 18, a gas delivery system 33, a remoteplasma cleaning system 50, a vacuum system 70, a source plasma system80A, a bias plasma system 80B.

The upper portion of chamber 13 includes a dome 14, which is made of aceramic dielectric material, such as aluminum oxide or aluminum nitride.Dome 14 defines an upper boundary of a plasma processing region 16.Plasma processing region 16 is bounded on the bottom by the uppersurface of a substrate 17 and a substrate support 18, which is also madefrom an aluminum oxide or aluminum ceramic material.

A heater plate 23 and a cold plate 24 surmount, and are thermallycoupled to, dome 14. Heater plate 23 and cold plate 24 allow control ofthe dome temperature to within about ±10° C. over a range of about 100°C. to 200° C. Generally, exposure to the plasma heats a substratepositioned on substrate support 18. Substrate support 18 includes innerand outer passages (not shown) that can deliver a heat transfer gas(sometimes referred to as a backside cooling gas) to the backside of thesubstrate.

The lower portion of chamber 13 includes a body member 22, which joinsthe chamber to the vacuum system. A base portion 21 of substrate support18 is mounted on, and forms a continuous inner surface with, body member22. Substrates are transferred into and out of chamber 13 by a robotblade (not shown) through an insertion/removal opening (not shown) inthe side of chamber 13. Lift pins (not shown) are raised and thenlowered under the control of a motor (also not shown) to move thesubstrate from the robot blade at an upper loading position 57 to alower processing position 56 in which the substrate is placed on asubstrate receiving portion 19 of substrate support 18. Substratereceiving portion 19 includes an electrostatic chuck 20 that can be usedto secure the substrate to substrate support 18 during substrateprocessing.

Vacuum system 70 includes throttle body 25, which houses twin-bladethrottle valve 26 and is attached to gate valve 27 and turbo-molecularpump 28. Gate valve 27 can isolate pump 28 from throttle body 25, andcan also control chamber pressure by restricting the exhaust flowcapacity when throttle valve 26 is fully open. The arrangement of thethrottle valve, gate valve, and turbo-molecular pump allow accurate andstable control of chamber pressures as low as about 1 mTorr.

Source plasma system 80A is coupled to a top coil 29 and side coil 30,mounted on dome 14. A symmetrical ground shield (not shown) reduceselectrical coupling between the coils. Top coil 29 is powered by topsource RF (SRF) generator 31A, whereas side coil 30 is powered by sideSRF generator 31B, allowing independent power levels and frequencies ofoperation for each coil. In a specific embodiment, the top source RFgenerator 31A provides up to 2,500 watts of RF power at nominally 2 MHzand the side source RF generator 31B provides up to 5,000 watts of RFpower at nominally 2 MHz. The operating frequencies of the top and sideRF generators may be offset from the nominal operating frequency (e.g.to 1.7-1.9 MHz and 1.9-2.1 MHz, respectively) to improveplasma-generation efficiency.

A bias plasma system 80B includes a bias RF (BRF) generator 31C and abias matching network 32C. The bias plasma system 80B capacitivelycouples substrate portion 17 to body member 22, which act ascomplimentary electrodes. The bias plasma system 80B serves to enhancethe transport of plasma species (e.g., ions) created by the sourceplasma system 80A to the surface of the substrate. In a specificembodiment, bias RF generator provides up to 5,000 watts of RF power at13.56 MHz.

RF generators 31A and 31B include digitally controlled synthesizers andoperate over a frequency range between about 1.8 to about 2.1 MHz. Eachgenerator includes an RF control circuit (not shown) that measuresreflected power from the chamber and coil back to the generator andadjusts the frequency of operation to obtain the lowest reflected power,as understood by a person of ordinary skill in the art. Matchingnetworks 32A and 32B match the output impedance of generators 31A and31B with their respective coils 29 and 30. The RF control circuit maytune both matching networks by changing the value of capacitors withinthe matching networks to match the generator to the load as the loadchanges. The RF control circuit may tune a matching network when thepower reflected from the load back to the generator exceeds a certainlimit. One way to provide a constant match, and effectively disable theRF control circuit from tuning the matching network, is to set thereflected power limit above any expected value of reflected power. Thismay help stabilize a plasma under some conditions by holding thematching network constant at its most recent condition.

A gas delivery system 33 provides gases from several sources 34(a) . . .34(n) via gas delivery lines 38 (only some of which are shown). In theparticular example illustrated below, gas sources 34(a) . . . 34(n)include separate sources for SiH₄, O₂, Ar and NF₃ as well as one or moresources for the extended cleaning process. As would be understood by aperson of skill in the art, the actual sources used for sources 34(a) .. . 34(n) and the actual connection of delivery lines 38 to chamber 13varies depending on the deposition and cleaning processes executedwithin chamber 13. Gas flow from each source 34(a) . . . 34(n) iscontrolled by one or more mass flow controllers (not shown) as is knownto those of skill in the art.

Gases are introduced into chamber 13 through a gas ring 37 and/or a topnozzle 45. FIG. 2B is a simplified, partial cross-sectional view ofchamber 13 showing additional details of gas ring 37. In someembodiments, one or more gas sources provide gas to ring plenum 36 ingas ring 37 via gas delivery lines 38 (only some of which are shown).Gas ring 37 has a plurality of gas nozzles 39 (only one of which isshown for purposes of illustration) that provides a uniform flow of gasover the substrate. Nozzle length and nozzle angle may be changed toallow tailoring of the uniformity profile and gas utilization efficiencyfor a particular process within an individual chamber. In one specificembodiment, gas ring 37 has 24 gas nozzles 39 made from an aluminumoxide ceramic.

Gas ring 37 also has a plurality of gas nozzles 40 (only one of which isshown), which in a specific embodiment are co-planar with and shorterthan source gas nozzles 39, and in one embodiment receive gas from bodyplenum 41. Gas nozzles 39 and 40 are not fluidly coupled in someembodiments where it is desirable to not mix gases (e.g., SiH₄ and O₂)introduced through gas ring 37 before injecting the gases into chamber13. In other embodiments, gases may be mixed prior to injecting thegases into chamber 13 by providing apertures (not shown) between bodyplenum 41 and gas ring plenum 36. Additional valves, such as 43B (othervalves not shown), may shut off gas from the flow controllers to thechamber.

In embodiments where flammable, toxic, or corrosive gases are used, itmay be desirable to eliminate gas remaining in the gas delivery linesafter a deposition or cleaning process. This may be accomplished using a3-way valve, such as valve 43B, to isolate chamber 13 from a deliveryline 38 and to vent delivery line 38 to vacuum foreline 44, for example.As shown in FIG. 2A, other similar valves, such as 43A and 43C, may beincorporated on other gas delivery lines. Such 3-way valves may beplaced as close to chamber 13 and remote plasma source 50 as practical,to minimize the volume of the unvented gas delivery line (between the3-way valve and the chamber). Additionally, two-way (on-off) valves (notshown) may be placed between a mass flow controller (“MFC”) and thechamber or between a gas source and an MFC.

Referring again to FIG. 2A, chamber 13 also has top nozzle 45 and topvent 46. Top nozzle 45 and top vent 46 allow independent control of topand side flows of the gases, which improves film uniformity and allowsfine adjustment of the film's deposition and doping parameters. Top vent46 is an annular opening around top nozzle 45. In one embodiment, onesource, e.g., SiH₄, supplies source gas nozzles 39 and top nozzle 45through separate MFCs (not shown). Similarly, separate MFCs may be usedto control the flow of oxygen to both top vent 46 and gas nozzles 40from a single source of oxygen. The gases supplied to top nozzle 45 andtop vent 46 may be kept separate prior to flowing the gases into chamber13, or the gases may be mixed in top plenum 48 before they flow intochamber 13. In other embodiments, separate sources of the same gas maybe used to supply various portions of the chamber.

A remote microwave-generated plasma cleaning system 50 is provided toperiodically clean deposition residues from chamber components in a drycleaning operation. The cleaning system includes a remote microwavegenerator 51 that creates a plasma from one or more cleaning gas sourcein sources 34(a) . . . 34(n) (e.g., molecular fluorine, nitrogentrifluoride, other fluorocarbons or equivalents alone or in combinationwith another gas such as Argon) in reactor cavity 53. The reactivespecies resulting from this plasma are conveyed to chamber 13 throughcleaning gas feed port 54 via applicator tube 55. The materials used tocontain the cleaning plasma (e.g., cavity 53 and applicator tube 55)must be resistant to attack by the plasma. The distance between reactorcavity 53 and feed port 54 should be kept as short as practical, sincethe concentration of desirable plasma species may decline with distancefrom reactor cavity 53. Generating the cleaning plasma in a remotecavity allows the use of an efficient microwave generator and does notsubject chamber components to the temperature, radiation, or bombardmentof the glow discharge that may be present in a plasma formed in situ.Consequently, relatively sensitive components, such as electrostaticchuck 20, do not need to be covered with a dummy wafer or otherwiseprotected, as may be required with an in situ plasma cleaning process.

System controller 60 controls the operation of system 10. Controller 60may include, for example, a memory 62, such as a hard disk drive and/ora floppy disk drive and a card rack coupled to a processor 61. The cardrack may contain a single-board computer (SBC), analog and digitalinput/output boards, interface boards and stepper motor controllerboards. System controller 60 operates under the control of a computerprogram stored on the hard disk drive or through other computerprograms, such as programs stored on a removable disk. The computerprogram dictates, for example, the timing, mixture of gases, RF powerlevels and other parameters of a particular process.

III. DEPOSITING A SILICON OXIDE FILM ACCORDING TO SPECIFIC EMBODIMENTSOF THE INVENTION

As previously stated, embodiments of the present invention can bepracticed in an HDP-CVD chamber such as exemplary chamber 13 describedabove. For convenience, one particular embodiment of the invention isdescribed with respect to FIG. 3. FIG. 3 is a flowchart illustratingvarious steps associated with the deposition of an undoped silicon oxidefilm (USG) according to one embodiment of the invention. The process isfor exemplary purposes only and is not intended to limit the scope ofthe claims of the present invention. Where applicable, reference numbersin the description below are used to refer to appropriate components ofthe exemplary chamber of FIGS. 2A-2B. In this embodiment the process isimplemented and controlled using a computer program stored in memory 62of system controller 60.

Referring to FIG. 3 the substrate upon which an undoped silicon oxidelayer is to be deposited according to the present invention istransferred into deposition chamber 13 (step 150). A flow of gas is thenintroduced into the chamber and a plasma is initiated to heat thesubstrate before deposition of the oxide film begins (step 155).Typically this heating step uses source RF power only (no bias RF power)in order to ensure the underlying substrate features are not sputtered.The substrate is typically heated to a temperature above 500° C. duringstep 155 but can be heated to even higher temperatures in someembodiments. In some embodiments, a heater within the chamber, e.g.,within the substrate support pedestal, can be used in addition to orinstead of the plasma to the substrate in step 155.

Typically the gas used to heat the substrate in step 155 does notinclude silicon. In one particular embodiment, the substrate is heatedby a plasma of argon and/or an oxygen source, such as O₂, for betweenapproximately 60-120 seconds.

Next, the plasma is extinguished and a deposition process gas is flowedinto the chamber in preparation for the film deposition step (step 160).The deposition process gas includes an oxygen source and a siliconsource. In one embodiment the oxygen source is O₂ and the silicon sourceis a silane gas such as SiH₄. After the flow of the process gas hasstabilized, high density a plasma is formed by applying RF power to sidecoil 30 and top coil (step 165). Power is also applied to chuck 20 tobias the plasma toward the substrate during step 155. In one particularembodiment, the plasma formed in step 165 is formed in a multistepprocess in which bias power is applied to the chuck, then the substrateis chucked while power is applied to the top coil and then finally tothe side coil.

The high density plasma is then maintained to deposit the silicon oxidefilm over the substrate (step 170). As previously stated, embodiments ofthe invention add a flow of helium to the process gas during step 170.In one embodiment, the flow of helium is added to the process gas instep 170 while in other embodiments the helium may be added in step 160or 165.

The inventors have discovered that using helium in the HDP silicon oxidedeposition process instead of argon may increase the gapfillcapabilities of the silicon oxide film. Helium, which is the secondlightest element in the periodic table, causes very limited sputteringduring the deposition process which is the primary source of sidewallredeposition. Helium is also characterized with its chemical inertness,high mobility and the highest ionization energy of its family (24.6 eVversus 15.8 eV for argon for the first degree of ionization) in a highdensity plasma. While not being limited to any particular theory, theinventors believe that the high mobility of helium, when introduced insufficient quantities, along with its long residence time limitsredeposition on the gap sidewalls. The inventors theorize that whenhelium enters the trench because of its long residence time it can stayon the sidewall for relatively long periods of time thereby preventingother material from contacting and adhering to the sidewall. Theinventors also theorize that helium may improve gapfill by increasingthe gas electron temperature thereby promoting a higher density plasma,which in turn allows silane and oxygen to become more easily ionized,and more ion directionality. By increasing ion directionality andlimiting sidewall redeposition, the deposition of the film within thegap is accelerated from the bottom up as opposed from the sidewall inresulting in improved gapfill.

The inventors have found that in order to achieve improved gapfill byadding helium to the process, it is important to flow a relatively highamount of helium into the chamber along with the silicon and oxygensources. To this end, the ratio of the flow rate of the helium source tothe combined flow rates of the oxygen and silicon sources should be atleast 0.5 to 1. At ratios below 0.5:1 the benefits of helium andimproved gapfill are not seen.

In some embodiments where an average silane flow (e.g., between 40-60sccm) is used, the ratio of the flow rate of the helium source to thecombined flow rates of the oxygen and silicon sources is preferably lessthan 3.0:1. When an average silane flow is used, the relatively high gasflow rates required to achieve ratios of helium to oxygen and siliconsources above 3.0:1 increases the chamber pressure to undesirably highlevels which in turn degrades film gapfill capabilities suppressing andeven overriding the benefits obtained by the addition of helium to theprocess gas. In other embodiments having an average silane flow rate,the ratio of helium flow rate to silane and oxygen flow rates is between1.5 and 2.5 to 1.

In still other embodiments though, where the rate at which silane isintroduced into the chamber is relatively low (e.g., between 15-30sccm), the helium to oxygen and silicon sources ratio may beconsiderably higher than 3.0:1. For example, in some embodiments, theratio can be 10:1 or higher. Reducing the flow rate of silane gasreduces the overall deposition rate of the silicon oxide film.Embodiments of the invention are able to deposit a silicon oxide film ata rate of between about 2500 to 3500 Å at average silane flow rates andat a rate of between about 1500 to 2100 Å at relatively low silane flowrates. Also, some embodiments of the invention employ an oxygen tosilane ratio of between 1.5-2.0:1 inclusive.

Various embodiments of the invention maintain pressure within thechamber during step 120 at a level of 7 mTorr or less and preferably ata pressure of 5 mTorr or less. Such relatively low pressure levels areimprove gapfill results by increasing the mean free path of ions anddissociated species thereby increasing the probability that suchdissociated species will travel to the bottom of the trenches to assistin a bottom-up deposition process. Pressure levels can be kept at aminimum level in some embodiments by fully opening the throttle valve sothat chamber pressure is controlled primarily by the flow rate of theprocess gas. Exemplary flow rates for the SiH₄, O₂ and He sources forsome embodiments are listed in Table 2 set forth below. TABLE 2EXEMPLARY PROCESS GAS FLOWS Total SiH₄ flow 39-90 sccm Total O₂ flow45-150 sccm Total He flow 100-400 sccm

After film deposition is complete, the flow of the silicon source isstopped, the substrate is dechucked and the plasma is continued for abrief time without bias power to purge the chamber (step 175). In someembodiments, only oxygen is flowed into chamber 13 during plasma purgestep 175. Other embodiments include helium or argon in the plasma.Finally, all gas flows are stopped and the substrate is transferred outof the chamber (step 180).

The method of the present invention is particularly useful for thedeposition of undoped silicon oxide layers for PMD and STI applications.Each of these applications often involve gapfill requirements that aremore aggressive, i.e., higher aspect ratio gaps, than IMD applications.Thus, the deposition process of many embodiments of the invention occursat a substrate temperature above 450° C. and more typically between 500°C. and 750° C. In embodiments of the invention used for IMDapplications, the substrate temperature is kept below 450° C.

FIG. 4 is a simplified cross-sectional view of a shallow trenchisolation (STI) structure formed in a semiconductor substrate 200 thatincludes the USG film deposited according to the process of FIG. 3. TheSTI structure shown in FIG. 4 includes a thin silicon nitride cap layer205 formed over raised surfaces 215 that define the trenches. A siliconoxide layer 220, deposited according to the present invention, fills thetrenches so that, preferably, no gap exists between adjacent raisedsurfaces.

In order to prove the effectiveness of the present invention, theinventors performed tests comparing the gapfill capabilities of filmsdeposited according to the present invention with films deposited usingthe prior art gapfill process shown in Table 1. These tests depositedundoped silicon oxide films over a silicon substrate having a pluralityof gaps having a width of 1.0 micron and an aspect ratio of 5:1 (ratioof height to width) etched therein. Table 3 set forth below shows thedeposition conditions used to test the method of the present invention.As with Table 1, when two numbers are listed for a gas flow variable,the first number is the flow from the side gas nozzle and the secondnumber is the flow from the top gas nozzle. TABLE 3 ONE EMBODIMENT OFTHE INVENTION Parameter Value SiH₄ flow 40 + 19 sccm O₂ flow 83 sccm Heflow 250 sccm Pressure 2-4 mTorr (TVO) Temperature >500° C. Top RF 6500Watts Side RF 6500 Watts Bias RF 2150 Watts

As a result of these tests, the inventors found that films depositedaccording to the process shown in Table 3 completely filled the highaspect ratio gaps while the films deposited according to the processshown in Table 1 left voids in the gaps.

The process parameters set forth above with respect to the experimentsand different embodiments are optimized for particular depositionprocesses run in an Ultima™ HDP chamber manufactured by AppliedMaterials that is outfitted for 200 mm wafers. A person of ordinaryskill in the art will recognize that these preferred parameters are inpart chamber specific and will vary if chambers of other design and/orvolume are employed.

Also, the parameters listed in the above embodiments and theabove-described experiments should not be limiting to the claims asdescribed herein. One of ordinary skill in the art can also useparameters and conditions other than those described with respect tospecific embodiments. For example, while the invention described withrespect to an undoped silicate glass layer, the invention can also beused to improve the gapfill capabilities of phosphosilicate glass (PSQ),boron-doped silicate glass (BSG) and boron phosphosilicate glass (BPGS)layers as well. For such films, a dopant gas such as PH₃ and/or B₂H₆ isadded to the process gas in step 170. Also, in other embodiments, anoxygen source such as N₂O or CO₂ can be used instead of O₂. As such, theabove description is illustrative and not restrictive. The scope of theinvention should, therefore, be determined not with reference to theabove description, but instead should be determined with reference tothe appended claims along with their full scope of equivalents.

1. A method for forming a silicon oxide layer over a substrate disposedin a high density plasma substrate processing chamber having a coiloperatively coupled to form a plasma within the chamber and bias plasmasystem, said method comprising: heating the substrate to a temperatureof at least 500° C. by introducing a first gas consisting of one or moreof argon and an oxygen-containing source into the substrate processingchamber and applying RF energy to the coil to form a plasma from thefirst gas without biasing the plasma toward the substrate; thereafter,flowing a process gas consisting of a silicon containing source, anoxygen-containing source and helium into the substrate processingchamber; wherein a ratio of a flow rate of said helium to a combinedflow rate of said silicon-containing source and said oxygen-containingsource is at least 0.5:1; and applying RF energy to the coil to form aplasma from said process gas in order to deposit said silicon oxidelayer, wherein said plasma ions/cm³ has an ion density of at least1.0×10¹¹.
 2. The method of claim 1 wherein said silicon-containingsource comprises monosilane (SiH₄).
 3. The method of claim 2 whereinsaid oxygen-containing source comprises molecular oxygen (O₂).
 4. Themethod of claim 3 wherein a ratio of said flow rate of said helium to acombined flow rate of said SiH₄ and said O₂ is between 1.5:1 and 2.5:1inclusive.
 5. The method of claim 4 wherein the oxide layer is depositedover a trench having a width of 0.1 μm or less and an aspect ratio of5:1 or higher.
 6. The method of claim 1 wherein a pressure level withinthe chamber is less than 7 mTorr during deposition of the silicon oxidelayer.
 7. The method of claim 6 wherein a ratio of said flow rate ofsaid helium to a combined flow rate of said SiH₄ and said O₂ is between0.5:1 and 3.0:1 inclusive.
 8. A method for forming a silicon oxide layerover a substrate, the method comprising: heating the substrate to atemperature of at least 500° C. by introducing a first gas consisting ofone or more of an inert gas and an oxygen source into the substrateprocessing chamber and applying RF energy to a coil operatively coupledto the chamber to form a plasma from the first gas without biasing theplasma toward the substrate; thereafter, depositing the silicon oxidelayer over the substrate by flowing a process gas comprising a siliconsource, an oxygen source and helium into the substrate processingchamber, applying RF energy to the coil to form a high density plasmafrom the process gas and biasing the high density plasma toward thesubstrate.
 9. The method set forth in claim 8 wherein the inert gas inthe first gas comprises argon.
 10. The method set forth in claim 8wherein the process gas further comprises a dopant and the silicon oxidelayer is a doped silicon oxide layer.
 11. The method set forth in claim8 wherein the process gas comprises a first flow of the silicon sourcethat is introduced into the chamber from positions around the peripheryof the substrate and a second flow of the silicon source that isintroduced into the chamber from a position over the substrate.
 12. Themethod set forth in claim 11 wherein the helium flow in the process gasis introduced into the chamber from positions around the periphery ofthe substrate.
 13. The method set forth in claim 12 wherein the siliconsource comprises monosilane (SiH₄) and the oxygen source comprisesmolecular oxygen (O₂).
 14. The method set forth in claim 8 wherein thesubstrate is chucked to a substrate support during deposition of thesilicon oxide layer and further comprising: after depositing the siliconoxide layer, dechucking the substrate from the substrate support whilemaintaining the plasma without biasing the plasma toward the substrateprior to stopping all the gas flow into the chamber and transferring thesubstrate out of the chamber.